About me

I'm a Ph.D. student in the UC Berkeley Department of Astronomy. I study the formation and evolution of galaxies using data from across the spectrum, optical to radio. Specifically, I use observations of post-starburst galaxies to investigate the physical mechanisms that cause galaxies to stop forming stars. My primary research advisor is Mariska Kriek. I publish under my full name, Katherine A. Suess.

I studied physics in undergrad at the University of Colorado, Boulder. I worked with Jeremy Darling to identify OH megamasers in the ALFALFA neutral hydrogen survey and with Marty Snow to model solar extreme ultraviolet irradiance.

My CV Publications (ADS)

My Research

I'm currently preoccupied with the question: what makes galaxies stop forming stars?
My research is aimed at understanding the physical mechanisms behind galaxy 'quenching,' the process by which disky blue star-forming galaxies evolve into elliptical red quiescent galaxies. I primarily study this transition by investigating post-starburst galaxies. These rare galaxies have just completed their major star-forming episode; since they represent the direct products of the quenching process, they're the ideal laboratory to understand what causes galaxies to quench.

Galaxies are smaller than they appear

We typically measure the sizes of galaxies by looking at their light profiles. But, depending on how galaxies grow and evolve over time, these light profiles may not be good tracers of the underlying mass profiles. In this paper, we used multi-band imaging of ~7,000 galaxies to measure their radial color gradients. We found that most galaxies are redder in the centers than in the outskirts, and so half-mass radii tend to be smaller than half-light radii. This changes both the normalization and the slope of the galaxy mass-size relation.

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Tons of fuel, but no 'fire'

Because stars form out of molecular gas, the amount of molecular gas a galaxy has is typically related to the number of new stars it's forming. Except for these galaxies! We looked at the molecular gas content of post-starburst galaxies at z~0.7. These galaxies have just finished their major star-forming episode, and don't appear to be forming many new stars. However, we found that these galaxies still contain large reservoirs of molecular gas. What is preventing the gas in these galaxies from forming stars? Is the star-formation just obscured in the wavelengths where we're observing? Follow-up projects to come...

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Imposters! Space lasers that masquerade as normal galaxies

Distant OH megamasers (giant space lasers that typically mark major galaxy mergers) can accidentally fall into blind spectroscopic surveys for local neutral hydrogen emitters-- the radio emission lines look just the same. In this study, which was my honors thesis project at CU Boulder, we found a method to distinguish OH megamasers and neutral hydrogen emitters based on their infrared colors. We also confirmed 127 ambiguous HI optical counterparts, discovered five new OH megamasers, and provided a way for future HI surveys to identify possible imposters without time-consuming, expensive optical spectroscopy.

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Outreach and Service

In addition to research, I love telling the public about astronomy. Find me at Cal Day, Grounds for Science, or Astro Night.

I'm also passionate about improving the climate in physics & astronomy. I'm currently the co-coordinator of Respect is Part of Research, a graduate student group that runs an annual peer-led sexual assault and sexual harassment prevention workshop for incoming first-year graduate students. Respect is Part of Research is actively growing, both at UC Berkeley and beyond-- please email me if you're interested in starting a similar peer-led workshop at your own institution.

Data & Downloads

Post-starburst galaxy sample

Here's the full post-starburst galaxy sample from Suess et al. 2017. The sample is selected from bright (i < 19), intermediate-redshift (z > 0.5), high-quality (median spectral S/N > 3.7) objects included in the full SDSS DR12 spectroscopic catalog (Alam et al. 2015). Post-starburst galaxies were selected from this parent sample according to the Kriek et al. 2010 method, which selects galaxies with "A-type" spectra by their strong Balmer breaks and their blue slopes redward of the break. We checked the final selection by eye to remove a few stars and brown dwarfs. In total, we found 50 post-starburst galaxies at 0.5 < z < 0.8. The text file below gives the IDs, redshifts, stellar masses, star-formation rates, and molecular gas masses for each galaxy in the sample. See Suess et al. 2017 for details on how the masses and SFRs listed here were calculated. IDs are 'spec' followed by the SDSS plate number, MJD, and fiber ID: these IDs can be used to query the SDSS database to see images, spectra, and photometry for each galaxy. If you use this sample, please cite Suess et al. 2017.

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